FIELD OF THE INVENTION
The present invention relates generally to electrical connectors for removably connecting an electrically-operated device to a box or receptacle connected in turn to a source of electrical power, and more particularly to a grounded and polarized plug and corresponding receptacle for use with theatrical, stage, and motion picture equipment such as high power state-of-the-art lighting equipment.
As alternating current (AC) power systems have largely replaced direct current (DC) power systems, the polarity of plugs and receptacles has become an important safety factor in the mechanical design of the plugs and receptacles. The convention in single phase AC electrical power is to have a "hot" wire and a "neutral" wire. The hot wire is connected to one side of a switch, with the other side of the switch being connected to the electrical device being operated. The neutral wire is connected directly to the electrical device.
When a receptacle is connected to a power source and a plug is used to connect the electrical device and the switch to the receptacle, it will be appreciated that a conventional unpolarized two prong plug may be inserted either way into a conventional unpolarized receptacle. This means that there is a fifty-fifty chance of inserting the plug into the receptacle with the wrong polarity, making the electrical device hot even when the switch is turned off. This is particularly serious with electrical devices which have what should be the neutral lead connected to the chassis or other metal parts of the device. The art has developed two ways of overcoming this problem, one with an independent ground and the other without.
The solution without using an independent ground is to design the plug and receptacle in a way allowing only one way of inserting the plug into a compatible receptacle. This may be accomplished by either aligning the prongs of the plug so as to allow only one way of inserting the plug into a compatible receptacle, or to make the prongs different sizes. The latter solution has found wide acceptance for use with 120V AC electrical power, with one of a pair of parallel prongs being wider than the other. A polarized plug and receptacle will ensure the correct polarity of the electrical appliance, thereby preventing possible electrical shock in electrical devices which have components which may be connected to the wire which is presumed to be the neutral wire. This solution is widely used with domestic lamps and with small appliances.
The solution using a ground pin has found use particularly with large electrical appliances, rather than with small electrical devices or with lights. A third prong is added to the hot prong and the neutral prong, which third prong is an independent ground. The third prong is typically round rather than flat, and is spaced away from and between the two typically parallel hot and neutral prongs. A variety of such ground pins are described in the art, such as for example in U.S. Pat. No. 3,733,576, to Cooper, in U.S. Pat. No. 3,739,317, to Wise, and in U.S. Pat. No. 3,786,392, to McDaniel. The use of a third wire to provide an independent ground will short circuit an inadvertent connection of the hot wire to the electrical device to the ground, thereby preventing the operator of the electrical device from receiving an electrical shock and possibly sustaining a severe physical injury.
The switch to AC from DC and the accompanying switch to the use of plugs and receptacles which are polarized, grounded, or both polarized and grounded has taken place in most parts of the world and in virtually all large electrical devices. One significant exception to this changeover has been in the lighting equipment and accessories which are used in the theatrical, stage, and motion picture industry. Historically, the lighting equipment has been powered by DC electrical power, with DCpowered equipment surviving the switch to AC electrical power by using large rectifiers supplied with DC power from conventional AC electrical power sources.
It will be recognized that the power requirements of the numerous large, high-powered lights used in the theatrical, stage, and motion picture industry are enormous, and that the plugs and receptacles used are likewise large and designed for making the connections between the high power DC supply and the large lighting loads. Stage plugs and receptacles, or "plugging boxes," as they are called, typically can make connections carrying 45 amps at 125 or 250 volts DC. Such receptacles or plugging boxes may be "one hole" (meaning only one stage plug may be inserted) or ganged "two hole," "four hole," or "six hole" portable plugging boxes in which two, four, or six stage plugs, respectively, may be inserted. In addition, fixed "stage pocket" receptacles are fixedly mounted in permanent locations on stages and in theaters for receiving stage plugs. Such stage plugs and their accompanying receptacles or plugging boxes, which are unpolarized and ungrounded, were designed for use with DC power.
Recently the art of lighting has made tremendous technological advances, with current state-of-the-art lighting being designed for use exclusively with AC electrical power. While the stage plugs and plugging boxes are in fact fairly safe when used with DC electrical power, they are absolutely unsafe and hazardous to life and property when used with AC electrical power. Controlling authorities from local communities and their authorities to the nationally known testing laboratories have been unanimous in prohibiting and decrying the use of the stage plugs and plugging boxes with AC electrical power.
This has presented the theatrical, stage, and motion picture industry with a choice between two poor alternatives- either use state-of-the-art lighting and buy polarized and grounded electrical cords and connectors not designed for use in the industry at a tremendous replacement cost, or stick with DC power supplies and the obsolete stage plugs and plugging boxes and use older, technologically obsolete lighting equipment. This is not a very palatable choice, and it makes apparent the need that exists for a way to utilize state-of-the-art AC powered lighting equipment with modified stage plugs and plugging boxes designed for use in the industry.
Accordingly, it is a primary object of the present invention to provide a stage plug and a plugging box for use with high power AC electrical lighting equipment which provides for safe use through both polarization and grounding of the equipment. Such a stage plug and plugging box must be acceptable to the regulating authorities for use with AC electrical equipment. It must also be comparatively simple in construction thereby making it both relatively inexpensive and easy to service, and be strong and durable yet compact and of light weight.
Perhaps most importantly, to provide a further economic incentive the parts of the stage plug and plugging box should be compatible insofar as possible with existing equipment, making it possible to modify existing stage plugs and plugging boxes to provide the ability to upgrade the existing equipment to a polarized, grounded configuration. Finally, a solution to the above-described problems must achieve all of the recited objectives and advantages without incurring any relative disadvantage, either in construction or in use.